The present invention relates to a novel process for preparing folic acid.
Robert B. Angier et al. (JACS, 70 (1948), 25) describe the preparation of folic acid using halogen-free compounds by reacting diethyl p-aminobenzoyl-L-glutamate with 2-hydroxymalonaldehyde, isolating diethyl p-(2,3-dihydroxypropenylideneamino)benzoylglutamate, and reacting the intermediate with triaminopyrimidone.
Other preparation methods are described in: O. Isler, G. Brubacher, S. Ghisla, B. Krxc3xa4utler, Vitamine II; G. Thieme Verlag Stuttgart (1988).
A low yield of folic acid is common to all these methods.
In addition, EP-A-0 608 693 describes a process for preparing folic acid in which 2-substituted malonaldehydes are reacted with p-aminobenzoyl-L-glutamic acid to form the corresponding diamine, which is reacted with triaminopyrimidone in the presence of sulfite to give folic acid. The disadvantage of this process is the difficulty of obtaining 2-substituted malonaldehyde.
It is an object of the present invention to provide a process for preparing folic acid with which folic acid is obtained in good yields by use of easily obtainable starting materials.
We have found that this object is achieved by a process for preparing folic acid, which comprises reacting a tetraalkoxypropanol of the general formula I, 
in which the substituents R are C1-C4-alkyl, with triaminopyrimidone of the formula II and p-aminobenzoyl-L-glutamic acid of the formula III 
Alkyl radicals for R mean C1-C4-alkyl radicals, for example methyl, ethyl, n-propyl, isopropyl or n-butyl, preferably methyl or ethyl, particularly preferably methyl.
It has now been found, surprisingly, that folic acid can be obtained in high yields by using tetraalkoxypropanol of the formula I, in particular tetramethoxypropanol which is easily obtainable industrially by electrochemical oxidation of methylglyoxal dimethyl acetal.
For the purpose of the process according to the invention, it is possible in the first reaction step to react the tetraalkoxypropanol both with triaminopyrimidone of the formula II and with the p-aminobenzoyl-L-glutamic acid of the formula III, the reaction advantageously taking place under acid conditions at a pH below 4 and at a temperature in the range from 0 to 100xc2x0 C.
The first reaction is preferably that of the tetraalkoxypropanol with the p-aminobenzoyl-L-glutamic acid of the formula III.
The reaction can take place in aqueous medium, where appropriate with the addition of inert, water-miscible, organic solvents such as acetonitrile, tetrahydrofuran, dimethylformamide, methanol, ethanol etc.
The acid reaction conditions can be adjusted, for example, by adding aqueous mineral acids such as aqueous hydrochloric acid, sulfuric acid, phosphoric acid, by adding organic C1-C4-carboxylic acids such as formic acid, acetic acid or propionic acid or by means of an acidic ion exchanger. Preferred acids in this connection are aqueous hydrochloric acid, formic acid and acetic acid.
Under the acidic hydrolysis conditions, the ketones of the general formula Ia 
in which the substituents R are C1-C4-alkyl, R1 is hydrogen or xe2x80x94C(xe2x95x90O)xe2x80x94R2 and R2 is hydrogen or C1-C3-alkyl, can be prepared from the corresponding compounds of the general formula I.
Alkyl radicals for R mean the C1-C4-alkyl radicals already mentioned above.
Alkyl radicals for R2 mean C1-C3-alkyl radicals such as methyl, ethyl, n-propyl or isopropyl.
Preferred radicals for R2 are hydrogen or methyl.
Very particularly preferred ketones of the formula Ia are those in which R is methyl and R1 is hydrogen or acetyl [xe2x80x94C(xe2x95x90O)xe2x80x94CH3].
The reaction, which has been mentioned at the outset and is preferably carried out, of tetraalkoxypropanol with p-aminobenzoyl-L-glutamic acid of the formula III can, under the abovementioned acidic conditions, lead in situ via the ketones of the general formula Ia to the intermediates of the general formula IV, 
in which the substituent Y is 
and R is C1-C4-alkyl, preferably methyl.
The reaction of compounds of the formula IV with triaminopyrimidone of the formula II advantageously takes place in the presence of sulfite, or inorganic compounds which form sulfites in water, at a pH of from 3 to 8 and temperatures of about 0 to 100xc2x0 C.
The employed sulfites or inorganic compounds which form sulfites in water are compounds such as Na2SO3, K2SO3, H2SO3, NaHSO3, Na2S2O5 or SO2 and the like. It is likewise possible to employ triaminopyrimidone sulfite.
The intermediates Ia and IV formed in the folic acid preparation according to the invention can be isolated after their preparation and then employed further in the process according to the invention. However, they can also be prepared in situ, and the reaction can accordingly also be carried out in a one-pot reaction.
The invention also relates to ketones of the general formula Ia 
in which the substituent R is C1-C4-Alkyl.
Preferred ketones of the general formula Ib are those in which the substituent R is methyl.
The invention likewise relates to compounds of the general formula IV 
in which the substituent Y is 
and R is C1-C4-alkyl, preferably methyl.